Nucleon spin physics with CLAS at Jlab

1
Nucleon spin physics with CLAS at Jlab
Marco Ripani INFN Genova, Italy For the EG1 group
and CLAS collaboration
Fifth International Conference on PERSPECTIVES IN
HADRONIC PHYSICS Particle-Nucleus and
Nucleus-Nucleus Scattering at Relativistic
Energies 22 - 26 May 2006, ICTP Trieste, Italy
2
Outline

• Physics Motivation
• Experimental Setup
• Nucleon Structure Functions
• Spin structure function and their x and Q2
  dependence
• Large-x behavior
• Sum rules, Moments and Higher Twists
• New low Q2 experiment

3
Spin Physics in the non-perturbative domain
Measurements of spin observables at large Q2
allowed the study of the spin dependence of
parton distribution functions (CERN, SLAC,
DESY) As Q2 decreases, non-perturbative effects
start to play a dominant role and the connection
between the observed nucleon properties and its
elementary constituent becomes highly non
trivial At low Q2, better description of the
nucleon properties can be obtained in terms of
hadronic degrees of freedom
Q2
gt 5 GeV2 ?1-2 GeV2 lt 1 GeV2
The study of the transition region between
hadronic and partonic degrees of freedom is a key
issue for the understanding of the nucleon
structure A broad program to study the nucleon
spin structure in the soft regime is in progress
in Hall B at Jefferson Lab with the goal of
mapping this kinematic region
4
Jefferson Lab

• CEBAF is a superconductive electron accelerator
• continuous beam
• high longitudinal polarization
• energy range ? 0.75 5.9 GeV
• current range ? 0.1 nA 200mA

The electron beam can be delivered simultaneously
to the three halls with high polarization
5
Experimental Setup
CEBAF Large Acceptance Spectrometer

• large kinematical coverage
• simultaneous measurement of
• exclusive and inclusive reactions
• central field-free region well suited
• for the insertion of the polarized
• target

6
Experimental Program

• measurement of the nucleon spin structure
  functions in the resonance region
• test of the generalized Gerasimov-Drell-Hearn
  Sum Rule on the proton and deuteron
• test of duality of spin structure function
• (see talk by P. Bosted)
• extraction of the moments of the proton and
  neutron structure functions and study of higher
  twist contribution
• study of the nucleon resonance structure from
  polarization observables in exclusive meson
  production
• measurement of spin asymmetries in semiinclusive
  processes
• deeply virtual compton scattering on polarized
  target

7
Spin Structure Functionsand their x and Q2
dependence
8
Asymmetries and Spin Structure Functions
A1
A2
e
qe
A1
A2
e
the structure functions A1 and A2 can be
extracted by varying the direction of the nucleon
polarization where D, ?, d, ? are function
of Q2, W, E0, R
Pe
Pt
y
nucleon
the structure functions g1 and g2 are linear
combination of A1 and A2
g1(x,Q2) g2(x,Q2)
9
A1hA2 for proton
1.7 GeV (proton)
5.7 GeV (proton)
Preliminary
Red solid line Parametrization of previous
world data, including CLAS data (S. Kuhn et al.
following original work from L. Stuart at
SLAC, further updated to include AO and
MAID2000 codes for resonance region) Blue solid
line Estimated contribution from the
unmeasured asymmetry A2 to the asymmetry A1hA2
10
g1 for the proton
Preliminary
Red solid line Parameterization of previous
world data, including CLAS data
11
Q2 dependence of g1/F1

• Q2 dependence of g1 at fixed x is very similar
  to F1 in the DIS region
• Our data show a
• decrease in g1/F1 even in the DIS region
• Resonance region ? different Q2
  dependence ? goes negative at D

Preliminary
12
Large x behavior
13
Large-x behavior of the A1 asymmetry

• SU(6) ?
• Hyperfine perturbed QM
• makes S1 pairs more energetic than S0 pairs
• At large x struck quark carries the spin of the
  nucleon
• Duality
• Suppress transitions to specific resonances (56
  and 70-)
• In DIS, and in PQCD
• Minimal gluon exchanges
• Spectator pair quarks have opposite helicities
• A1 ?1

Isgur, PRD 59, 034013 (2003)
Close and Melnitchouk, PRC 68, 035210 (2003)
Farrar and Jackson, PRL 35, 1416 (1975)
14
Large-x behavior of A1
Close and Melnitchouk, PRC 68, 035210 (2003)
Isgur, PRD 59, 034013 (2003)

W gt 2 GeV
W gt 2 GeV
Proton
Deuteron

• P and d results fall below parameterization of
  world data at 10 GeV2 ? include in DGLAP fits
• To be used to extract Dq/q in this momentum
  transfer region (see talk by M. Garçon)
• P and d results are in better agreement with the
  HFP quark model

15
Sum Rules, Momentsand Higher Twists
16
GDH Sum Rule
IGDH

• relates the difference of the photo-absorption
  cross section for helicity 1/2 and 3/2 to the
  nucleon magnetic moment, i.e. a connection
  between dynamic and static properties. Recent
  measurements at Bonn and Mainz, ongoing efforts
  at other labs
• based on very general principles, as gauge
  invariance, dispersion relation, low energy
  theorem
• at finite Q2 can be related to
• the integral of the spin structure
• function g1
• strong variation of nucleon spin
• properties as a function of Q2

G1
DIS pQCD
Transition Region
GDH sum rule
17
Generalized GDH Integral
A generalization of the GDH sum rule has been
suggested by Ji and Osborne by relating the
virtual-photon forward Compton amplitude S1 to
the nucleon structure function G1 X.Ji et al.,
Phys.Lett.B472 (2000) 1
GDH sum rule and Chiral Perturbation Theory -
cQ2 O (Q4) Lattice? Ellis-Jaffe Sum
rule and Operator Product Expansion ?
Q2 lt 0.1 GeV2
S1 (0,Q2)
? 2 M2
? x0
ò
16pa Q2
IGDH (Q2) dx g1(x,Q2)
0.1 lt Q2 lt 0.5 GeV2
Q2 gt 0.5 GeV2
the left side is a calculable quantity the right
side is a measurable quantity
mt(Q2) Qt-2
t2,4,
18
Integral of g1 on the Proton

• the integral is consistent with previous SLAC
  data
• shows strong Q2 dependence varying from negative
  to positive values as Q2 increases
• change in slope occurs at 0.15 GeV2

Preliminary
19
Integral of g1 on the Deuteron

• consistent with previous SLAC data
• slower transition than for the proton
• change in slope occurs at 0.2 GeV2

Preliminary
20
Bjorken Sum Rule

• combined analysis of Hall A and Hall B
  measurements
• A. Deur et al., PRL 93 212001(2004) and new
  preliminary analysis (A. Deur et al.)
• consistent with previous SLAC data
• D and other isospin 3/2 contribution cancel out
• better agreement with cPT than for separated
  proton and neutron integrals

Preliminary
21
Moments and Higher Twists
Nachtmann moments (Leading twist is
incorporating correctly kinematic twists)

• New global extraction of g1 structure functions
  and analysis in terms of OPE ? Higher Twist
  extraction
• M. Osipenko et al.,
• PRD 71, 054007(2005)
• PLB 609, 259(2005)

a3, a8 taken from b decay
More in talk by W. Melnitchouk
22
First moment and HT
High Q2 fit
Low Q2 fit
? Higher twists small
23
Color polarizabilities

• Disagrees with QCD sum rules, instanton vacuum
  model
• Consistent with MIT bag model

24
New low Q2 measurement
25
New Experiments
New completed experiment E-03-006 The GDH
Sum Rule with nearly real photons and
the proton g1 structure function at low momentum
transfer New Proposals P-05-111
Measurement of the Gerasimov-Drell-Hearn Integral
at low Q2on the Neutron and Deuteron P-05-113
Semi-Inclusive Pion Production with a
Longitudinally Polarized Target at 6 GeV
P-05-114 Deeply Virtual Compton Scattering at
6 GeV with polarized target and polarized
beam using the CLAS detector
26
Proton Structure Function at Very Low Q2

• Extension of previous experiments
• Test of cPT at Q2? 0
• New Cerenkov Counter to detect scattered elecrons
  down to 6 deg. (INFN-Genova)

Q2 (GeV2)
W (GeV)
27
Proton Structure Function at Very Low Q2
expected results for statistical errors on top of
THEORETICAL cross section difference based on S.
Simulas parameterization, S. Simula et al., PRD
65, 034017 (2002)
IGDH
Three sample points from actual
experiment (different horizontal scale)
From proposal
Q2 (GeV2)
28
Summary

• A wealth of new data on the nucleon spin
  structure in the non-perturbative regime has
  been produced in Hall B at Jefferson Lab as part
  of a broad spin physics program, still in
  progress
• These measurements provide new information for
  understanding the transition between hadronic and
  partonic degrees of freedom by investigating spin
  structure functions, related sum rules and
  moments, asymmetries,
• A new measurement to cover the very low momentum
  transfer region and provide a bridge to the GDH
  sum rule at the photon point has just been
  successfully completed

29
Integral of g1 on the Proton

• the integral is consistent with previous SLAC
  data
• shows strong Q2 dependence varying from negative
  to positive values as Q2 increases
• change in slope occurs at 0.15 GeV2

30
Integral of g1 on the Deuteron

• consistent with previous SLAC data
• slower transition than for the proton
• change in slope occurs at 0.2 GeV2